This invention relates to an antenna construction for a radio transmitter or receiver to be carried on the wrist, and more particularly relates to an expansion band antenna for a wristwatch radio device.
Several proposals are known for antennas for small portable radios in which the antenna is incorporated into a belt or strap which also supports the radio on the person of the user. Examples of these are shown in U.S. Pat. No. 2,470,687 issued to Cafrella et al. on May 17, 1949, U.S. Pat. No. 3,523,296 issued to Vliegenthardt on Aug. 4, 1970, U.S. Pat. No. 2,255,897 to Rebori et al. on Sept. 16, 1941, and U.S. Pat. No. 4,340,972 issued to Heist on July 20, 1982. The Heist and Vliegenthardt patents depict antennas designed to function as conventional dipoles. The Cafrella et al. patent shows a loop antenna stitched between two plies of a supporting belt, and the Rebori patent depicts a loop antenna with a parallel connected tuning capacitor and coupled to a crystal "detector".
Proposals are also known for combining a radio transmitter or receiver with a timepiece and arranging the antenna for the transmitter or receiver inside two separate halves of a wristband, the conductors in each half being connected to the radio device inside the timepiece case. An example is shown in U.S. Pat. No. 3,032,651 to Gisiger-Stahli et al. on May 1, 1962 having serpentine conductors folded back and forth longitudinally along the halves. Another proposal appears in published European patent application No. 0 100 639 A2 published Feb. 15, 1984 in the name of Sinclair Research Limited. A continuous watchband is shown with transversely oriented loops strung on a pair of conductors running longitudinally and embedded in the watchband, the separate loops being wound on ferrite cores.
A proposal for a wristwatch receiver antenna is disclosed in PCT application, International Publication No. WO 86/03645 published June 19, 1986 in the name of AT&E Corporation, in which the watchband comprises two sections of a strip conductor within a strap fastened by a conductive clasp or buckle. This construction requires special grommets on one side to make connection with the tongue of the buckle on the other side or use of a conductive clasp. Such proposals introduce the possibility of electrical discontinuities the midpoint of the antenna. An alternate proposal in the aforesaid application was to zig-zag a conductor through successive links of a metal expansion band. The AT&E construction attaches the strap or band ends to conventional spring bars which also are electrically connected to the antenna ends. One spring bar makes electrical connection to the case through spring contact. The other spring bar makes spring contact with inner cylindrical members which are connected to the radio receiver. Outer cylindrical members serve as capacitors with the inner cylindrical members to tune the antenna. The use of several pressure or spring loaded connections create electrical discontinuities which can be a source of noise.
Normally an antenna is designed with regard to the wavelength at which it is to be operated. However, a wristwatch antenna is obliged to transfer energy within the constraints of the physical size of the wrist instrument. The theory of small antennas is set forth in Small Antennas by Harold A. Wheeler published in IEEE Transactions and Antennas and Propogation, volume AP-23, No. 4, July 1975 and also in an article entitled "Loop Antennas" by Glenn S. Smith, pages 5-2 through 5-9 appearing in Antenna Engineering Handbook, Second Edition, published by McGraw Hill, 1984. As the antenna is made smaller, the most important effect on its performance is the decrease in its radiation resistance, a measure of the amount of energy transmitted by an antenna, which in turn decrease its efficiency: therefore, an antenna for a wrist instrument must be made as large as possible. The greatest physical dimension of a member which is available as the antenna for a wrist instrument is the circumference of the wrist, which is typically around 20 cm. For a resonant loop antenna, the circumference of the loop is equal to half a wavelength. This implies that a loop around the wrist would be resonant at approximately 750 MHz. If the wrist loop antenna is operated at frequencies below half its self resonant frequency, the radiation resistance can range from microohms to a few ohms for frequencies ranging from 40 to 500 MHz. Since the efficiency of the antenna is a transmitter is the ratio of the radiation resistance to the total resistance of the antenna, the total resistance must be kept low. The total resistance includes the resistance of the conductor, contact points and the grounding system.
When designing the antenna for a receiver, one of the most important factors that must be considered, is its effect on the range of the instrument. The range of any receiver is greatly affected by the amount of electrical noise it adds to the incoming signal. Non-permanent connections, such as screwed-down connections or other types of pressure contact points which are not permanently soldered in place are such a source of electrical noise.
One of the requirements for a wrist instrument is to be able to get the instrument off and on the wrist. This either requires a buckle or clasp, or an expansion band. An expansion band will permit an antenna construction with a single conductor without electrical discontinuities which might degrade its performance as an antenna and does not require that a buckle or clasp be reconnected if the instrument is to be operated after it has been removed from the wrist. However, an expansion band must be flexible and able to expand and contract without affecting the performance of the small antenna associated with it.
Accordingly, one object of the present invention is to provide an improved expansion band antenna for a wristwatch transmitter/receiver.
Another object of the invention is to provide an improved loop antenna for a wrist instrument which efficiently operates as a tuned circuit for radio frequency transmission or reception.
Another object of the invention is to provide an improved antenna for a wrist instrument with a minimum number of pressure contacts or spring loaded electrical connections.